1. Field of the Invention
This invention relates to a spark plug.
2. Description of the Related Art
A spark plug used for ignition of an internal engine of such as automobiles generally comprises a metal shell to which a ground electrode is fixed, an insulator made of alumina ceramics, and a center electrode which is disposed inside the insulator. The insulator projects from the rear opening of the metal shell in the axial direction. A terminal metal fixture is inserted into the projecting part of the insulator and is connected to the center electrode via a conductive glass seal layer which is formed by a glass sealing procedure or a resistor. A high voltage is applied to the terminal metal fixture to cause a spark over the gap between the ground electrode and the center electrode.
Under some combined conditions, for example, at an increased spark plug temperature and an increased environmental humidity, it may happen that high voltage application fails to cause a spark over the gap but, instead, a discharge called as a flashover occurs between the terminal metal fixture and the metal shell, going around the projecting insulator. Primarily for the purpose of avoiding flashover, most of commonly used spark plugs have a glaze layer on the surface of the insulator. The glaze layer also serves to smoothen the insulator surface thereby preventing contamination and to enhance the chemical or mechanical strength of the insulator.
In the case of the alumina insulator for the spark plug, such a glaze of lead silicate glass has conventionally been used where silicate glass is mixed with a relatively large amount of PbO to lower a softening point. In recent years, however, with a globally increasing concern about environmental conservation, glazes containing Pb have been losing acceptance. In the automobile industry, for instance, where spark plugs find a huge demand, it has been a subject of study to phase out Pb glazes in a future, taking into consideration the adverse influences of waste spark plugs on the environment.
Leadless borosilicate glass- or alkaline borosilicate glass-based glazes have been studied as substitutes for the conventional Pb glazes, but they inevitably have inconveniences such as a high glass transition or an insufficient insulation resistance. To address this problem, JP-A-11-43351 proposes a leadless glaze composition having an adjusted Zn component to improve glass stability without increasing viscosity, and JP-A-11-106234 discloses a composition of leadless glaze for improving the insulation resistance by effects of joint addition of alkaline component.
The glaze layer for the spark plug not only prevents the insulator surface from adhering of dirt or stain, heightens withstand voltage of creeping discharge to prevent flashover, but also serves to bury defects in the insulator surface which are apt to cause a destruction starting point for increasing strength. However, in recent internal combustion engines remarkable in high output, vibration and impact received by the spark plug during working, so that problems often occur as breakage of the insulator though being formed with the glaze layer. In addition, when attaching the spark plug to a cylinder head (in particular when attaching with power tools such as impact wrench), if adding over tightening torque, the insulator will be broken. Further, since voltage applied to the spark plug is getting higher accompanied with high performance of engines, the glaze has been demanded to have an insulating performance durable against severe circumstances, but compositions of the glaze disclosed in JP-A-11-106234 or JP-A-11-43351 are involved with problems that the glaze compositions compatible in the insulation performance and mechanical properties are not always investigated.
It is accordingly an object of the invention to provide spark plugs having such glaze layers containing less Pb component, enabling to be baked at relatively low temperatures, having excellent insulating property, easily realizing smooth baked surfaces, and heightening the mechanical strength of the insulator with the glaze layer.
For solving the above problems, the spark plug of the invention has an insulator comprising alumina based ceramic disposed between a center electrode and a metal shell, wherein at least part of the surface of the insulator is covered with a glaze layer comprising oxides, and is characterized
in that the glaze layer comprises
Pb component 1 mol % or less in terms of PbO;
Si component 15 to 60 mol % in terms of SiO2;
B component 22 to 50 mol % in terms of B2O3;
Zn component 10 to 30 mol % in terms of ZnO;
Ba and/or Sr components 0.5 to 35 mol % in terms of BaO or SrO;
F component 1 mol % or less,
Al component 0.1 to 5 mol % in terms of Al2O3; and
alkaline metal components of 5 to 10 mol % in total of one kind or more of Na, K and Li in terms of Na2O, K2O, and Li2, respectively, where Li is essential, and the amount of the Li component is 1.1 to 6 mol % in terms of Li2O.
In the spark plug according to the invention, for aiming at the adaptability to the environmental problems, it is a premise that the glaze to be used contains the Pb component 1.0 mol % or less in terms of PbO (hereafter called the glaze containing the Pb component reduced to this level as xe2x80x9cleadless glazexe2x80x9d) When the Pb component is present in the glaze in the form of an ion of lower valency (e.g., Pb2+), it is oxidized to an ion of higher valency (e.g., Pb3+) by a corona discharge. If this happens, the insulating properties of the glaze layer are reduced, which probably spoils an anti-flashover. From this viewpoint, too, the limited Pb content is beneficial. A preferred Pb content is 0.1 mol % or less. It is most preferred for the glaze to contain substantially no Pb (except a trace amount of lead unavoidably incorporated from raw materials of the glaze).
However, according to an inventor""s studies, it was proved that if the amount of Pb component was smaller, a mechanical strength of the glaze layer, in particular impact resistance was apt to relatively decrease. Therefore, it was found that if Si, B, Zn, Ea and/or Sr, and Al components, further alkaline metal component were contained in the above mentioned range, such glaze layers could be provided, enabling to be baked at relatively low temperatures, having excellent insulating property, easily realizing smooth baked surfaces, and heightening the mechanical strength, especially the impact resistance of the insulator formed with the glaze layer, and thus the present invention has been accomplished. Thereby, in case the spark plug is attached to the high output internal combustion engine, the insulator of the spark plug is unlikely to break by such as vibrations during working. Further, if tightening torque somewhat exceeds when attaching the spark plug to the cylinder head (especially when attaching with power tools such as an impact wrench), the insulator is unlikely to break.
In the following, reference will be made to critical meanings of ranges containing respective composing components of the glaze layer in the present spark plug. Si component is a skeleton forming component of the glaze layer of vitreous substance, and is indispensable for securing the insulating property. With respect to the Si component, being less than 15 mol %, it is often difficult to secure a sufficient insulating performance. Being more than 60 mol %, it is often difficult to bake the glaze. The Si containing amount should be more preferably 25 to 40 mol %.
B component is also a skeleton forming component of the glaze layer of vitreous substance, and if combined with Si a skeleton forming component of the glaze layer of vitreous substance, a softening point of the glaze is lowered and fluidity when baking the glaze is improved for easily obtaining smooth baked surfaces. If the B containing amount is less than 20 mol %, the softening point of the glaze goes up, and the baking of the glaze will be difficult. On the other hand, being more than 55 mol %, inferior external appearance such as a glaze crimping is easily caused. Or, water-proof might be spoiled. Depending on containing amounts of other components, such apprehensions might occur as a devitrification the glaze layer, the lowering of the insulating property, or inconsequence of the thermal expansion coefficient in relation with the substrate. It is good to determine the B containing amount to range 25 to 35 mol % if possible.
Zn component heightens the fluidity when baking the glaze in substitution for Pb component for easily obtaining the smooth baked surfaces. If compounding Zn component more than a predetermined amount, difference in coefficient of thermal expansion between a substrate of the insulator of alumina based ceramic and the glaze layer is reduced to prevent occurrence of defects in the glaze layer and to restrain residual level of tension residual stress, and heighten strength of the insulator formed with the glaze layer, in particular the impact resistance. If the Zn containing amount is less than 10 molt, the thermal expansion coefficient of the glaze layer is too large, defects such as crazing are easily occur in the glaze layer. As the Zn component acts to lower the softening point of the glaze, if it is short, the baking of the glaze will be difficult. Being more than 30 mol %, opacity easily occurs in the glaze layer due to the devitrification. It is good that the Zn containing amount to determine 10 to 20 mol %.
Ba and Sr components contribute to heightening of the insulating property of the glaze layer and is effective to increasing of the strength. If the total amount is less than 0.5 mol %, the insulating property of the glaze layer goes down, and the anti-flashover might be spoiled. Being more than31 mol %, the thermal expansion coefficient of the glaze layer is too high, defects such as crazing are easily occur in the glaze layer. Tension stress is easy to remain in the glaze layer when cooling from high temperatures, and strength of the insulator formed with the glaze layer, e.g., the impact resistance is easily spoiled. In addition, the opacity easily occurs in the glaze layer. From the viewpoint of heightening the insulating property and adjusting the thermal expansion coefficient, the total amount of Ba and Sr is desirably determined to be 0.5 to 20 mol %, and in particular if the Si component ranges 25 to 40 mol %, the effect is large. Either or both of the Ba and Sr component may be contained, but the Ba component is advantageously cheaper in a cost of a raw material.
The Ba and Sr components may exist in forms other than oxides in the glaze depending on raw materials to be used. For example, BaSO4 is used as a source of the Ba component, an S component might be residual in the glaze layer. This sulfur component is concentrated nearly to the surface of the glaze layer when baking the glaze to lower the surface expansion of a melted glaze and to heighten a smoothness of a glaze layer to be obtained.
A reason for F component to be 1 mol % or lower is why if the glaze contains F component of more than 1 mol % (if adding into the glaze, e.g., a catalyst containing F component such as CaF2 (fluorite), p component is inevitably mixed), air bubbles are ready for a rising which are easy to cause breakdown in the glaze when baking it, this attributes to spoiling of the strength of the insulator having the glaze layer, for example, the impact resistance. Further, a gas bearing F component issues when baking the glaze, and this trends to invite inconveniences of reacting with a refractory composing an oven wall to shorten the life of the oven wall. More desirably, F component is not contained in the glaze layer if possible, and it is better not to use the catalyst containing F component as CaF2 if circumstances allow.
Al component broadens a temperature range available for baking the glaze, stabilizes the fluidity when baking the glaze, and largely heightens the impact resistance of the insulator formed with the glaze. But if being less than 0.1 mol % in terms of oxide, the effect thereof lacks. Further, if being over 5 mol %, the glaze layer to be produced is opaque and mat, and the external appearance of the spark plug is spoiled, and markings formed on the substrate are illegible, resulting in inconveniences as when de-vitrifying. The amount of Al component is desirably 1 to 3 mol %.
Next, the alkaline metal components in the glaze layer is mainly used to lower the softening point of the glaze layer and to heighten the fluidity when baking the glaze. The total amount thereof is determined to be 1.1 to 10 mol %. In case of being less than 1.1 mol %, the softening point of the glaze goes up, baking of the glaze might be probably impossible. In case of being more than 10 mol %, the insulating property probably goes down, and an anti-flashover might be spoiled. The containing amount of the alkaline metal components is preferably 5 to 8 mol %. With respect to the alkaline metal components, not depending on one kind, but adding in joint two kinds or more selected from Na, K and Li, the insulating property of the glaze layer is more effectively restrained from lowering. As a result, the amount of the alkaline metal components can be increased without decreasing the insulating property, consequently it is possible to concurrently attain the two purposes of securing the fluidity when baking the glaze and the anti-flashover (so-called alkaline joint addition effect).
Among the above mentioned alkaline metal components, Li component has particularly high effect for improving the fluidity when baking the glaze, and is not only useful for obtaining the baked smooth surface with lesser defects but also remarkably effective for suppressing increase of the thermal expansion coefficient, and considerably controls tension residual stress appearing in the glaze layer. Each of these effects displays to improve strength of the insulator with the glaze layer, for example, the impact resistance. If being less than 1.1 mol % in terms of oxide of Li component, the effect is poor, and being more than 6 mol %, the insulating property of the glaze layer is not sufficiently secured. The amount of Li component is desirably 2 to 4 mol %.
Further reference will be made to desirable compositions of the glaze layer.
It is desirable that the glaze layer contains Zn component of NZnO (mol %) in terms of ZnO, Ba component of NBaO (mol %) in terms of BaO, and Sr component of NSrO (mol %) in terms of SrO, and the total amount of NZnO+NBaO+NSrO is 15 to 45 mol %. If exceeding 45 mol %, the glaze layer will be devitrified and slightly opaque. For example, on the outer surface of the insulator, visual information such as letters, figures or product numbers are printed and baked with color glazes for identifying makers and others, and owing to the slight opaqueness, the printed visual information is sometimes illegible.
Or, if being less than 15 mol %, the softening point exceedingly goes up to make the glaze baking difficult and cause bad external appearance. Thus, the total amount is more desirably 15 to 25 mol %.
The glaze layer is preferably to be NZnO greater than NBaO+NSrO. Thereby, it is possible to make the thermal expansion coefficient of the glaze layer smaller, more shorten the difference in the thermal expansion coefficient from alumina based ceramic to be the substrate to reduce the tension stress level remaining in the glaze layer after baking, and moreover to bring the residual stress under a condition of compressive stress. As a result, the impact resistance of the glaze layer can be more heightened.
It is desirable that Li component is determined to be in a range of 0.2xe2x89xa6Li/(Na+K+Li)xe2x89xa60.5 in mol % in terms of oxides as above mentioned. If being less than 0.2, the thermal expansion coefficient is too large in comparison with alumina of the substrate, and consequently, defects such as crazing are easy to occur and finishing of the baked glaze surface is insufficiently secured. On the other hand, if being more than 0.5, since Li ion is relatively high in migration among alkaline metal ions, bad influences might be affected to insulating property of the glaze layer. Values of Li/(Na+K+Li) are more desirably adjusted to be 0.3 to 0.45. For more heightening the effect of improving the insulating property, it is possible to compound other alkaline metal components than third components such as K, Na and subsequent components in ranges of not spoiling the effect of controlling conductivity by excessive co-addition of alkaline metal component. Especially desirably, the three components are all contained.
Further, it is preferable that the glaze layer satisfies that NB2O3/(NZnO+NBaO+NSrO) is 0.5 to 2.0. Being less than 0.5, the glaze layer is easily de-vitrified, and being over 2.0, the softening point of the glaze layer goes up to make sometimes the glaze baking difficult.
It is possible to contain one kind or more of Ti, Zr and Hf 0.5 to 5 mol % in total in terms of ZrO2, TiO2 and HfO2.
By containing one kind or more of Ti, Zr or Hf, a water resistance is improved. As to the Zr or Hf components, the improved effect of the water resistance of the glaze layer is more noticeable. By the way, xe2x80x9cthe water resistance is goodxe2x80x9d is meant that if, for example, a powder like raw material of the glaze is mixed together with a solvent as water and is left as a glaze slurry for a long time, such inconvenience is difficult to occur as increasing a viscosity of the glaze slurry owing to elusion of the component. As a result, in case of coating the glaze slurry to the insulator, optimization of a coating thickness is easy and unevenness in thickness is reduced. Subsequently, said optimization and said reduction can be effectively attained. If being less than 0.5 mol %, the effect is poor, and if being more than 5 mol %, the glaze layer is ready for devitrification.
It is possible to contain Mo, W, Ni, Co, Fe and Mn (called as xe2x80x9cfluidity improving transition metal componentxe2x80x9d hereafter) 0.5 to 5 mol % in total in terms of MoO3, WO3, Ni3O4, Co3O4, Fe2O3, and MnO2, respectively. If adding one kind or more of Mo, W, Ni, Co, Fe and Mn in the above mentioned containing range, it is possible to secure the fluidity when baking the glaze. Therefore, the glaze layer having the excellent insulating property can be obtained by baking at relatively low temperatures. Due to the baked smooth surface, the impact resistance of the insulator with the glaze layer thereon can be heightened further.
If the total amount in terms of oxides is less than 0.5 mol %, it may be difficult to obtain a sufficient effect of improving the fluidity when baking the glaze and of easily obtaining a smooth glaze layer. On the other hand, if exceeding 5 mol %, it may be difficult or impossible to bake the glaze owing to an excessive rise of the softening point of the glaze.
When the containing amount of the fluidity improving transition metal component is excessive, coloring may unintentionally appear in the glaze layer. For example, visual information such as letters, figures or product numbers are printed with color glazes on external appearances of the insulators for specifying manufacturers and others. However, if the colors of the glaze layer is too thick, it might be difficult to read out the printed visual information through the glaze layer. As another realistic problem, there is a case that tint changing resulted from alternation in the glaze composition is seen to purchasers as xe2x80x9cunreasonable alternation in familiar colors in external appearancexe2x80x9d, so that an inconvenience occurs that products could not always be willingly accepted because of a resistant feeling thereto.
The insulator forming a substrate of the glaze layer comprises alumina based ceramics taking white, and in view of preventing or restraining coloration, it is desirable that the coloration in an observed external appearance of the glaze layer formed in the insulator is adjusted to be 0 to 6 in chroma Cs and 7.5 to 10 in lightness Vs, for example, the amount of the above transition metal component is adjusted. If the chroma of the glaze layer exceeds 6, the coloration of the glaze layer is remarkably perceived. On the other hand, if the lightness is less than 7.5, the gray or blackish coloration is easily perceived. In either way, there appears a problem that an impression of xe2x80x9capparent colorationxe2x80x9d cannot be prevented. The chroma Cs is preferably 8 to 10, more preferably 9 to 10. In the present specification, a measuring method of the lightness Vs and the chroma Cs adopts the method specified in xe2x80x9c4.3 A Measuring Method of Reflected Objectsxe2x80x9d of xe2x80x9c4. Spectral Colorimetryxe2x80x9d in the xe2x80x9cA Measuring Method of Colorsxe2x80x9d of JIS-Z8722 (1994). And the result measured by the above method is compared with standard color chart prepared according to JIS-Z8721 to know the lightness and the chroma.
As a simple substitutive method, the lightness and the chroma can be known just through visual comparisons with standard color chart prepared according to JIS-Z8721 (1993).
The effect of improving the fluidity when baking the glaze is remarkably exhibited by W next to Mo and Fe. For example, it is possible that all the essential transition metal components are made Mo, Fe or W. For more heightening the effect of improving the fluidity when baking the glaze, it is preferable that Mo is 50 mol % or more of the fluidity improving transition metal components.
The glaze layer may contain one or two kinds of Ca component of 1 to 10 mol % in terms of CaO and Mg component of 0.1 to 10 mol % in terms of MgO in the total amount of 1 to 12 mol %. These components contribute to improvement of the insulating property of the glaze layer. Especially, Ca component is effective next to Ba component and Zn component, aiming at improvement of the insulating property. If the addition amount is less than their lower limits, the effective may be poor, or exceeding their upper limits or the upper limit of the total amount, the baking glaze may be difficult or impossible owing to excessive increase of the softening point.
Auxiliary components of one kind or more of Bi, Sn, Sb, P, Cu, Ce and Cr may be contained 5 mol % or less in total as Bi in terms of Bi2O3, Sn in terms of SnO2, Sn in terms of Sb2O5, P in terms of P2O5, Cu in terms of CuO, Ce in terms of CeO2, and Cr in terms of Cr2O3. These components may be positively added in response to purposes or often inevitably included as raw materials of the glaze (otherwise later mentioned clay minerals to be mixed when preparing a glaze slurry) or impurities (otherwise contaminants) from refractory materials in the melting procedure for producing glaze frit. Each of them heightens the fluidity when baking the glaze, restrains bubble formation in the glaze layer, or wraps adhered materials on the baked glaze surface so as to prevent abnormal projections. Bi and Sb are especially effective.
In the composition of the spark plug of the invention, the respective components in the glaze are contained in the forms of oxides in many cases, and owing to factors forming amorphous and vitreous phases, existing forms as oxides cannot be often identified. In such cases, if the containing amounts of components at values in terms of oxides fall in the above mentioned ranges, it is regarded that they belong to the ranges of the invention.
The containing amounts of the respective components in the glaze layer formed on the insulator can be identified by use of known micro-analyzing methods such as EPMA (electronic probe micro-analysis) or XPS (X-ray photoelectron spectroscopy) For example, if using EPMA, either of a wavelength dispersion system and an energy dispersion system is sufficient for measuring characteristic X-ray. Further, there is a method where the glaze layer is peeled from the insulator and is subjected to a chemical analysis or a gas analysis for identifying the composition.
If the above mentioned composition is employed for the glaze layer, taking, as a backward direction, a side remote from spark discharge gap in an axial direction of the insulator, the metal shell is fixed such that the backward part of the insulator projecting from the metal shell is perpendicular with respect to a test article securing bed, while an arm of 330 mm length furnished at the front end with a steel made hammer of 1.13 kg is turnably attached to an axial fulcrum located on a center axial line of the insulator at a more upper part of the backward part of the insulator, and a location of the axial fulcrum is determined such that a position of the hammer when it is brought down onto the backward part of the insulator is 1 mm as a distance in the vertical direction from the backward face of the insulator, the hammer is brought up such that a turning angle of the arm is at predetermined angle from the center axial line, and when operation of bringing down the hammer owing to free dropping toward the backward part of the insulator is repeated as stepwise making larger at distance of 2 degree, impact endurance angle demanded as a limit angle when cracks appear in the insulator is 35 degree or more. Thereby, even if vibration/impact are received, or when the spark plug is attached to the high output internal combustion engine or to the cylinder head (especially when attaching with power tools such as an impact wrench), even if tightening torque somewhat exceeds, the insulator is effectively restrained from breakdown.
The insulator is formed with a projection part in an outer circumferential direction at an axially central position thereof. Taking, as a front side, a side directing toward the front end of the center electrode in the axial direction, a cylindrical face is shaped in the outer circumferential face at the base portion of the insulator main body in the neighborhood of a rear side opposite the projection part. In this case, the outer circumferential face at the base portion is covered with the glaze layer formed with the film thickness ranging 7 to 50 xcexcm.
In automobile engines, such a practice is broadly adopted that the spark plug is attached to engine electric equipment system by means of rubber caps, and for heightening the anti- flashover, important is the adherence between the insulator and the inside of the rubber cap. The inventors made earnest studies and found that, in the leadless glaze of borosilicate glass or alkaline borosilicate, it is important to adjust thickness of the glaze layer for obtaining a smooth surface of the baked glaze, and as the outer circumference of the base portion of the insulator main body particularly requires the adherence with the rubber cap, unless appropriate adjustment is made to the film thickness, a sufficient anti-flashover cannot be secured. Therefore, in the insulator having the leadless glaze layer of the above mentioned composition of the spark plug according to the third invention, if the film thickness of the glaze layer covering the outer circumference of the base portion of the insulator is set in the range of the above numerical values, the adherence between the baked glaze face and the rubber cap may be heightened, and in turn the anti-flashover may be improved without lowering the insulating property of the glaze layer.
By adjusting the thickness of the glaze layer as mentioned above, the impact resistance of the insulator formed with the glaze layer can be more improved. If the thickness of the glaze layer at said portion of the insulator is less than 7 xcexcm, the anti-flashover property is insufficient, otherwise the glaze layer is too thin, so that an absolute strength or a defect covering effect in the insulator surface is not enough, and the impact resistance is short. On the other hand, if the thickness of the glaze layer exceeds 50 xcexcm, it is difficult to secure the insulator with the leadless glaze layer of the above mentioned composition, similarly resulting in decrease of the anti-flashover or resulting in too much increase after baking the glaze of the residual stress amount to be determined with balance between the thermal expansion rate and the thickness of the glaze layer so that the impact resistance might lack. The thickness of the glaze layer is desirably 10 to 30 xcexcm.
The spark plug having the glaze layer of the invention may be composed by furnishing, in a crazing hole of the insulator, an axially shaped terminal metal fixture as one body with the center electrode or holding a conductive binding layer in relation therewith, said metal fixture being separate from a center electrode. In this case, the whole of the spark plug is kept at around 500xc2x0 C., and an electric conductivity is made between the terminal metal fixture and a metal shell, enabling to measure the insulating resistant value. For securing an insulating endurance at high temperatures, it is desirable that the insulating resistant value is secured 200 Mxcexa9 or higher so as to prevent the flashover.
FIG. 4 shows one example of measuring system. That is, DC constant voltage source (e.g., source voltage 1000 V) is connected to a terminal metal 13 of the spark plug 100, while at the same time, the metal shell 1 is grounded, and a current is passed under a condition where the spark plug 100 disposed in a heating oven is heated at 500xc2x0 C. For example, imagining that a current value Im is measured by use of a current measuring resistance (resistance value Rm) at the voltage VS, an insulation resistance value Rx to be measured can be obtained as (VS/Im)xe2x88x92Rm (in the drawing, the current value Im is measured by output of a differential amplifier for amplifying voltage difference at both ends of the current measuring resistance).
The insulator may include the alumina insulating material containing the Al component 85 to 98 mol % in terms of Al2O3. Preferably, the glaze layer has an average thermal expansion coefficient of 50xc3x9710xe2x88x927/xc2x0 C. to 85xc3x9710xe2x88x927/xc2x0 C. at the temperature ranging 20 to 350xc2x0 C. Being less than this lower limit, defects such as cracking or graze skipping easily happen in the graze layer. On the other hand, being more than the upper limit, defects such as crazing are easy to happen in the graze layer. The thermal expansion coefficient more preferably ranges 60xc3x9710xe2x88x927/xc2x0 C. to 80xc3x9710xe2x88x927/xc2x0 C.
The thermal expansion coefficient of the glaze layer is assumed in such ways that samples are cut out from a vitreous glaze bulk body prepared by mixing and melting raw materials such that almost the same composition as the glaze layer is realized, and values measured by a known dilatometer method.
The thermal expansion coefficient of the glaze layer on the insulator can be measured by use of, e.g., a laser interferometer or an interatomic force microscope.
The spark plug of the invention can be produced by a production method comprising
a step of preparing glaze powders in which the raw material powders are mixed at a predetermined ratio, the mixture is heated 1000 to 1500xc2x0 C. and melted, the melted material is rapidly cooled, vitrified and ground into powder;
a step of piling the glaze powder on the surface of an insulator to form a glaze powder layer; and
a step of heating the insulator, thereby to bake the glaze powder layer on the surface of the insulator.
The powdered raw material of each component includes not only an oxide thereof (sufficient with complex oxide) but also other inorganic materials such as hydroxide, carbonate, chloride, sulfate, nitrate, or phosphate. These inorganic materials should be those of capable of being converted to corresponding oxides by heating and melting. The rapidly cooling can be carried out by throwing the melt into a water or atomizing the melt onto the surface of a cooling roll for obtaining flakes.
The glaze powder is dispersed into the water or solvent, so that it can be used as a glaze slurry. For example, if coating the glaze slurry onto the insulator surface to dry it, the piled layer of the glaze powder can be formed as a coated layer of the glaze slurry. By the way, as the method of coating the glaze slurry on the insulator surface, if adopting a method of spraying from an atomizing nozzle onto the insulator surface, the piled layer in uniform thickness of the glaze powder can be easily formed and an adjustment of the coated thickness is easy.
The glaze slurry can contain an adequate amount of a clay mineral or an organic binder for heightening a shape retention of the piled layer of the glaze powder. As the clay mineral, those comprising mainly aluminosolicate hydrates can be applied, for example, those comprising mainly one kind or more of allophane, imogolite, hisingerite, smectite, kaolinite, halloysite, montmorillonite, vermiculite, and dolomite (or mixtures thereof) can be used. In relation with the oxide components, in addition to SiO2 and Al2O3, those mainly containing one kind or more of Fe2O3, TiO2, CaO, MgO, Na2O and K2O can be used.
The spark plug of the invention is constructed of an insulator having a through-hole formed in the axial direction thereof, a terminal metal fixture fitted in one end of the through-hole, and a center electrode fitted in the other end. The terminal metal fixture and the center electrode are electrically connected via an electrically conductive sintered body mainly comprising a mixture of a glass and a conductive material (e.g., a conductive glass seal or a resistor). The spark plug having such a structure can be made by a process including the following steps.
An assembly step: a step of assembling a structure comprising the insulator having the through-hole, the terminal metal fixture fitted in one end of the through-hole, the center electrode fitted in the other end, and a filled layer formed between the terminal metal fixture and the center electrode, which filled layer comprises the glass powder and the conductive material powder.
A glaze baking step: a step of heating the assembled structure formed with the piled layer of the glaze powder on the surface of the insulator at temperature ranging 800 to 950xc2x0 C. to bake the piled layer of the glaze powder on the surface of the insulator so as to form a glaze layer, and at the same time softening the glass powder in the filled layer.
A pressing step: a step of bringing the center electrode and the terminal metal fixture relatively close within the through-hole, thereby pressing the filled layer between the center electrode and the terminal metal fixture into the electrically conductive sintered body.
In this case, the terminal metal fixture and the center electrode are electrically connected by the electrically conductive sintered body to concurrently seal the gap between the inside of the through-hole and the terminal metal fixture and the center electrode. Therefore, the glaze baking step also serves as a glass sealing step. This process is efficient in that the glass sealing and the glaze baking are performed simultaneously. Since the above mentioned glaze allows the baking temperature to be lower to 800 to 950xc2x0 C., the center electrode and the terminal metal fixture hardly suffer from bad production owing to oxidation so that the yield of the spark plug is heightened. It is also sufficient that the baking glaze step is preceded to the glass sealing step.
The softening point of the glaze layer is preferably adjusted to range, e.g., 520 to 700xc2x0 C. When the softening point is higher than 700xc2x0 C., the baking temperature above 950xc2x0 C. will be required to carry out both baking and glass sealing, which may accelerate oxidation of the center electrode and the terminal metal fixture. When the softening point is lower than 520xc2x0 C., the glaze baking temperature should be set lower than 800xc2x0 C. In this case, the glass used in the conductive sintered body must have a low softening point in order to secure a satisfactory glass seal. As a result, when an accomplished spark plug is used for a long time in a relatively high temperature environment, the glass in the conductive sintered body is liable to denaturalization, and where, for example, the conductive sintered body comprises a resistor, the denaturalization of the glass tends to result in deterioration of the performance such as a life under load. Incidentally, the softening point of the glaze is adjusted at temperature range of 520 to 620xc2x0 C.
The softening point of the glaze layer is a value measured by performing a differential thermal analysis on the glaze layer peeled off from the insulator and heated, and it is obtained as a temperature of a peak appearing next to a first endothermic peak (that the second endothermic peak) which is indicative of a sag point. The softening point of the glaze layer formed in the surface of the insulator can be also estimated from a value obtained with a glass sample which is prepared by compounding raw materials so as to give substantially the same composition as the glaze layer under analysis, melting the composition and rapidly cooling.